NPN AND Gate Schematic

In this AND Gate that was made using NPN transistors, the LED is only illuminated when both buttons are pressed. By doing so, some current flows into the base pin of each transistor which allows for the majority of current to flow through them from collector to emitter. This completes the circuit and thus turns the LED on. If only one button is pressed the circuit is still not complete, both must be pressed to pass this logic gate.